Magnetic recording apparatus



Dec. 21, 1954 JEAN-JACQUES BESSIRE 2,697,756

MAGNETIC RECORDING APPARATUS Filed Nov. 7, 1951 Fig. 2a

Flg. 2b

United States Patent 6 MAGNETIC RECORDING APPARATUS Jean-Jacques Bessire, Bienne, Switzerland, assignor to Produits Perfectone S. A., Bienne, Switzerland, a corporation of Switzerland Application November 7, 1951, Serial No. 255,273

Claims priority, application Switzerland November 8, 1950 3 Claims. (c1. 179-10o.z

This invention relates to magnetic sound recording and play-back apparatus, and more especially to a device for improving the reproduction of sounds by means of mag netized sound carriers.

When playing back sound recorded on a magnetized carrier, strong linear distortions occur in consequence of the potential induced in the pick-up rising linearily with the frequency. On the other hand, during sound recording the magnetic field intensity set up is practically in dependent from the frequency and proportional to the amplitude of a tone. Therefore the amplifiers of the magnetic sound devices must possess a frequency characteristic decreasing in proportion as the frequency rises.

This is true up to a certain point where owing to dispersion in the magnetic sound carrier and to an upper frequency limit created by the width of the air gap in the pick-up, the potential generated in these elements drops rapidly. Therefore, in order to widen upwardly the entire frequency band of the device the amplifier must be able to produce a rise of the amplification at the upper end of the frequency curve in order to compensate for the drop of the potential induced in the picloup.

It is an object of this invention to produce in a simple manner the desired rise of amplification in devices of the kind here in view.

The invention thus comprises a correcting system characterized in that the induction coil in the pick-up forms, together with a condenser connected in parallel to it, a parallel oscillating circuit in the grid circuit of the playback amplifiers first amplifying stage which is arranged for negative feed-back, or counter-coupled, by means of a network between the anode and the grid.

In the drawing afiixed to this specification and forming part thereof Fig. 1 illustrates an embodiment of this invention in a purely diagrammatical manner,

Fig. 2a is a curve showing the frequency response of the magnetic head, and

Fig. 2b is a curve showing the frequency response of the amplifier network according to this invention.

In the drawing 1 is the magnetic sound carrier which is passed across the pole shoes 2 of a pick-up 3. In the induction coil 4 alternating voltages are generated which are fed to the grid of the first amplifier tube 5 of the playback amplifier through a resistance 6. A condenser 7 is connected in parallel to the induction coil 4. The anode of the tube 5 is connected to the grid of the tube 5 by means of a network comprising the resistances 8 and 9 and the condensers 10 and 11. The capacity 7 is so chosen that the resonance frequency of the oscillating circuit which the capacity forms with the induction coil 4,

the amplification of the tube 5 is great.

lies about in the middle of the steep drop of the voltage curve of the sound head.

The effects produced by this oscillating circuit and by the negative feed-back network assist one another in producing the desired amplifier characteristic.

Owing to the presence of the condenser 10 the negative feed-back coetiicient is very small for low frequencies and In proportion as the frequency rises, the negative feed-back coefficient rises also. Apart from this, the conductance of the network connected in parallel to the outer resistance Ra rises. Both these facts cause the amplification of the tube 5 to drop towards the high frequencies. The condenser 11 further has the effect that in proportion to the rise of frequency the phase of the negative feed-back network rises also. Near the resonance of the oscillating circuit 4, 7 the potential supplied by the pick-up rises considerably provided the amplitude of the inducing magnet field remains constant, and at the point of resonance reaches a maximum. This, however, is also the region where the oscillating circuit exerts a decisive influence on the phase conditions of the negative feed-back network, so that the negative feed-back is changed into a reaction coupling and hereby the drop of the output potential at high frequencies can be delayed.

Obviously the negative feed-back network should be so organized that no self-excitation arises in the system.

The invention thus allows considerably improving the frequency characteristic of any existing magnetic sound recording and play-back device merely by adding some simple circuit elements.

Various changes may be made in the details of the invention hereabove described and illustrated in the drawing without departing from the invention or sacrificing the advantages thereof.

I claim:

1. In a magnetic sound recording and play-back apparatus comprising an induction coil pick-up, a condenser shunted across the terminals of said induction coil and a play-back amplifier, a circuit arrangement for correcting the frequency characteristics of said sound recording and play-back apparatus, said arrangement consisting in that the oscillatory circuit formed with said induction coil and said condenser is coupled to the circuit of the first amplifying stage through a reversed feed-back network connecting the anode and the grid of said amplifier, said reversed feed-back network being frequency responsive and consisting of an RC-circuit.

2. The circuit arrangement of claim 1, in which the resonance frequency of the oscillatory circuit lies near the limiting frequency of the pick-up system.

3. The circuit arrangement of claim 1, in which the reversed feed-back network is so chosen that the reverse feed-back is ineffective near the upper limiting frequency of the pick-up system.

References Cited in the file of this patent FOREIGN PATENTS Country Date Germany Sept. 23, 1933 OTHER REFERENCES Number 

